The purpose of this investigation is to attempt to explain the alterations of kidney transamidinase activities found in rats in various dietary and hormonal states. Kidneys from the following rats have only a fraction of the normal transamidinase activities: 1) rats fed a protein-free diet or a complete purified diet supplemented with either creatine of guanidinoacetic acid; 2) hypophysectomized or thyroidectomized rats; 3) intact rats given large doses of either the adrenal steroids, the female steroid hormones or the thyroid hormones. hypophysectomized or thyroidectomized rats that have received injections of growth hormone or thyroxine respectively have normal kidney transamidinase activities. Rats fed complete diets supplemented with either creatine or quanidinoacetic acid plus glycine have much greater kidney transamidinase activities than found in rats fed the same diets without glycine. Since no cofactors or activators have been found or implicated for rat kidney transamidinase, the above listed alterations of enzyme activities seem likely to be the result of changes in rates of synthesis or breakdown of the enzyme or alterations in the enzyme composition or properties. We believe it is now possible to investigate this problem. A procedure for the isolation of rat kidney transamidinase has been developed in our laboratory. The production of antibodies to this enzyme should now be possible. Rats in various hormonal and dietary states will be given injections of labeled amino acids and sacrificed at varying times after the injections. The combination of immunochemical and isotopic techniques should make possible the determination of alterations in rates of synthesis or breakdown of the enzyme. Also, purified transamidinase from the rats in various hormonal and dietary states will be examined for composition or conformation differences.